Optical time domain reflectometry (OTDR) is a well-known technique for measuring the distribution of a number of parameters of an optical fiber, such as attenuation, core diameter, numerical aperture, and even chromatic dispersion. When a narrow-band source is used in an optical time domain reflectometer to interrogate an optical fiber, Rayleigh backscattered light is produced in response to an interrogating pulse launched into the fiber. In essence, the interrogating pulse can be thought of occupying a certain length of the fiber and, assuming that the pulse is coherent, all the electric dipoles arising from the non-homogeneity of the glass have a fixed (though random) phase relationship to one another. The resulting backscatter signal for a particular section of the fiber is then treated as the coherent sum of all the electric fields of these dipoles. This sum is, of course, dependent on the phase as well as the amplitudes of each dipole. For a fixed optical source frequency and a fixed state of the fiber (i.e., a fixed temperature, strain, etc.), the backscatter return (relative to the pulse energy) from a particular location is fixed, but randomly related to the backscatter return from any other section of fiber. As used herein, the backscatter return is the optical power (or intensity) as detected, for example, by a photodiode generally located at the launching end of the fiber. The detected backscatter signature for a given state of the fiber takes the form of a spiky waveform, with excursions about a mean value of at least a factor of ten. When the state of the fiber is varied, the vector summation changes and the backscatter return thus changes. These changes in the detected backscatter signal may be exploited for detecting disturbances, such as intruders into a perimeter monitored by the fiber, or even for measuring dynamic strain (i.e., changes in a strain level without particular interest in the absolute strain value). While, conceptually, these effects are easiest thought of in single-mode fibers, the following description is not restricted to single-mode fibers and indeed similar effects have been observed in multimode fibers.
In some applications, tunable optical sources have been used to scan the optical carrier frequency while performing reflectometric measurements in optical fibers. However, while such applications typically provide high resolution in the measurement of a parameter of interest, there is generally either no position resolution (i.e., the measurement is a pure optical frequency domain reflectometry (OFDR) measurement), or the technique has been used to measure discrete reflectors, such as fiber Bragg gratings, as opposed to a distributed measurement. Moreover, such applications typically compare a signal from a reference fiber to a signal from the fiber of interest. The use of a reference fiber and the comparison involved in such applications adds an unnecessary level of complexity that detracts from the measurement of absolute values of the distribution of the parameter of interest.